I experience this too. Although, actually picturing the act of doing it step by step, the possible pain and fear during the process. The possibility of it failing- the reality of it effectively, still frightens me.
For me, I think it's partly the rebelious side to it that makes it exciting. An ultimate 'fu*k this' to life and everything that was/ is expected of me. I sometimes get the weirdest tingling sensation in my left hand when I think about suicide.
I'm not meaning to glorify it here. This is simply a description of my own feelings towards it.
I also think it's a weird form of motivation to do it. I don't think many would deny that suicide is frightening. In order to push myself to potentially hurt myself and cause myself an unknown amount of pain and fear- which I'm naturally reluctant to do, I think my brain realises it will need to latch on to a whole raft of emotions to push through that. (SI)
For me personally, it will likely be fear of my life and all it involves plus my likely deteriorating future. Excitement that my problems are hopefully coming to an end. Plus, I'll likely try to convince myself that I'm a strong person who will take risks when required.